Send content-type: text/plain when erroring out 'cause we can't write to disk

core/01-settings.fragment.php

@@ -37,6 +37,7 @@ if(!file_exists($settingsFilename)) {
 	$settings->secret = bin2hex(random_bytes(16));
 	if(file_put_contents("peppermint.json", json_encode($settings, JSON_PRETTY_PRINT)) === false) {
 		http_response_code(503);
+		header("content-type: text/plain");
 		exit("Oops! It looks like $settings->sitename wasn't able to write peppermint.json to disk.\nThis file contains all of $settings->sitename's settings, so it's really important!\nHave you checked that PHP has write access to the directory that index.php is located in (and all it's contents and subdirectories)? Try\n\nsudo chown USERNAME:USERNAME -R path/to/directory\n\nand\n\nsudo chmod -R 0644 path/to/directory;\nsudo chmod -R +X path/too/directory\n\n....where USERNAME is the username that the PHP process is running under.");
 	}
 }